Negative or positive wash-off relief systems

ABSTRACT

A wash-off relief system in which the residual portion of the light-sensitive gel coating exposed to a copy image may be either the light-struck (negative image) portion or the non-light-struck (positive image), at the option of the user. The coating includes a mixture of ferric double salts which, alone, normally do not insolubilize the gel coating, and ferric single salts which normally insolubilize the gel coating. To obtain a continous coating, the mixed salts are dispersed in the gel material in the presence of a volatile solubilizer (acid) sufficient to offset the insolubilizing effect of the ferric single salts, the solubilizer leaving the film coating during drying to provide a substantially insolubilized light-sensitive gel coating prior to exposure. Negative images are developed in a developer solution containing hydrogen peroxide and an acid (or a peroxide former and an acid) prior to wash-off. Positive images may be left by wash-off without use of a developer, but in some formulations, a positive developer solution containing salts of acids which solubilize the gel coating and salts of alkaline earth metals (particularly strontium and barium) may be used to aid wash-off.

United States Patent [1 1 McIntosh [11] 3,804,621 Apr. 16, 1974 NEGATIVE OR POSITIVE WASH-OFF RELIEF SYSTEMS [75] Inventor: Maurice D. McIntosh, Willoughby,

Ohio

[73] Assignee: M. D. McIntoch & C0., Willoughby,

Ohio

[22] Filed: May 25, 1967 [21] App]. No.: 641,113

OTHER PUBLICATIONS T. Baker, American Photography, Oct. 1940, pp. 718-720, 721, 722, 724, J. Kosar, Light Sensitive Systems, John Wiley & Sons, N.Y., 1966; pp. 39, 40, ll4-l l6 relied upon.

Primary Examiner--J. Travis Brown Assistant ExaminerM. F. Kelley Attorney, Agent, or Firm--Ely and Golrick 5 7] ABSTRACT A wash-off relief system in which the residual portion of the light-sensitive gel coating; exposed to a copy image may be either the light-struck (negative image) portion or the non-light-struck (positive image), at the option of the user. The coating includes a mixture of ferric double salts which, alone, normally do not insolubilize the gel coating, and ferric single salts which normally insolubilize the gel coating. To obtain a continous coating, the mixed salts are dispersed in the gel material in the presence of a volatile solubilizer (acid) sufficient to offset the insolubilizing effect of the ferric single salts, the solubilizer leaving the film coating during drying to provide a substantially insolubilized light-sensitive gel coating prior to exposure.

Negative images are developed in a developer solution containing hydrogen peroxide and an acid (or a peroxide former and an acid) prior to wash-off. Positive images may be left by wash-off without use of a developer, but in some formulations, a positive developer solution containing salts of acids which solubilize the gel coating and salts of alkaline earth metals (particularly strontium and barium) may be used to aid wash-off.

12 Claims, N0 Drawings NEGATIVE on POSITIVE WASH-OFF RELIEF SYSTEMS This invention relates to so-called wash-off relief systems used for preparing stencils for the screens employed in screen-printing; for preparing guide-line color separation plates in maps and similar multi-color printing in which the colors are printed non-tonally; proofing systems in color printing; and the like. More particularly, this invention relates to films or plates usable in such systems which may produce either positive or negative images, at the option of the user to meet his particular need, and the methods of making and using such films or plates.

Essentially such wash-off relief systems have heretofore employed an organic gel coating on a relatively dimensionally-stable transparent support and depend on the facts that (a) if the gel, particularly animal gelatin, contains certain double or complex ferric salts, it may be soluble or rendered insoluble in water, and .(b) these ferric salts, particularly double-salts of a base and a mild organic acid, are light-sensitive when dispersed in such age] and the iron therein may be reduced to the ferrous state upon exposure to light. A specific example of such a wash-ofi relief system comprises coating a transparent film (or;plate) withan aqueoussolutionof gelatin and ferric ammonium citrate, one of the double ferric salts which does not i nsolu'bilize "the gelatin; the coating usually .containsan inert pigment :or eo'lor toner which enables the "final image ,to be examined visually and which imay also he required forsome z partiou'lar-end uses. Afterthe coated film isdr'ied,' itlis exposed tollight which is projected as an image onto the film :or by contact printed through a transparency or translucency carrying an original .or copy of the image to be reproduced. The exposed film is then immersed in an aqueous solution of hydrogen peroxide (or peroxide formers, such as sodium rperborate and a mild acid). This insolu'bilizes the lightwex posed I areas of the gelatin coating and allowsthe unexposed areas-of the coatingto be washed off the supportingifilm .or plate with' warm water. After being so developed, the residual reliefimage of insolubilized gelatin may be put to a variety of uses. For example, if a supporting film or plate is provided with a suitable Lrelease between itand the layer ofgelatin, or is not particularly adherent to the gelatin 'layer, the developedimage of insolubilized I gelatin may be adhered or otherwise transferred, by stripping the relief image off its original supportyto the screen of ascreen- Iprinting stenciL where it serves-to=close offthe:area'of the screen through which ink is 'stenciled. Or, if the color toners in the gelatin relief images have been selected "to match *the inks intended to be Iused in multicolor printingand *the "images have been developed to correspond to the images in color separation printing plates, assembly of a series of such -wash-off relief imagesand "projection of light through them will produce a colored projection which can serve as a I proof of the plates and *indicate where =make-1ready corrections on theprinting plates or in'fthetoning of'the inks should be image has been required, it has been necessary either to impose a negative copy image, as by projection through a photographic negative of the wash-off relief image desired, or. to develop a second or final positive wash-off relief image from an initial negative wash-off relief. By either of these alternatives, material costs, processing labor, and, most importantly, processing time is greatly increased, reducing or eliminating the advantages in speed, simplicity, and cost of wash-off relief systems over other competitive systems for obtaining the required image.

"Various wash-off relief systems which, theoretically, will produce directpositive images have been proposed, but they have enjoyed little, if any, practical or commercial success; in general they have been very slow and have not had the reliability of the negativeimage systems. Further a very practical and heretofore inescapable fault has been that, if a .given user had .a sufficient demand for both positive and negative images or a variation in the form in which the original copy was supplied to him '(i.e., as positive or negative images), to vvarrantconsidering use of a direct positive wash-off relief film at all, nevertheless he would have to carry an inventory of both films .(or plates) which would provide direct negative images and those which would provide direct positive images. 'When coupled with the slowness of such direct positive systems as were available heretofore (as much as minutes process'ing time in some systemstas compared with direct negative systems (about 2 minutes processing time) and the need for different processing techniques and materials, the need for maintaining a double inventory of materials, if one were to make directpositives as well as direct negatives, has heretofore confined the practical tusenof wash-off 1reliefsystemslargely to direct negative image systems only.

Accordingly, an object and advantage .of this invention is to :provide a washsoff relief system which may ,produce, :from a single exposed .coated film or plate, eiither a developed negative or a developed positive wash-off relief image, at the option of the user, of the light image imposed upon the film or plate.

-Another0bject and advantage of the invention is to provide a direct positive image wash-off relief system which is as fast andureliableaswthe directnegative image systems -'Iheretofore available.

A' furtherobject and advantage of this invention is to provide a tougher and more durable negative relief image than has been aheretofore obtainable, regardless of whether :the .relief image is a negative or positive of the original copy. *In the .case .of negative images, this is obtained withoutxloss or I increased complexity of processing Itime.

Other. objects and advantages of this invention will be apparent from the -following detailed description and mixture ofiferric'double salts which do notinsolubilize .the. gel andferric single salts whieh normally doinsolubilizethe-gel, irrespective of exposure to light.

Due tothe .presenceof the insolubilizing single salts which wotildnormally be expectedl to precipitate the. gel from solution and prevent it fromformingtherequisite .continuous Ifilm and, instead, leave at best a gummy, rubbery mass, alternative procedures may be employed to provide the requisite coating of a gel containing a mixture of single and double ferric salts. The preferable procedure is to prepare an aqueous coating solution of the gel material, usually gelatin, and the well-known light-sensitive double ferric salts which do not insolubilizc the gelatin; examples of such salts are ferric ammonium citrate, ferric ammonium oxalate, and ferric ammonium tartrate (the latter being operable, but barely so); one may also use such double organic acid salts in which the ammonium is replaced with a monovalent alkali metal (e.g., sodium, potassium, and lithium) such as, for example, ferric sodium, potassium, or lithium citrate or oxalate. To such a solution is added one or more single ferric salts which normally insolubilizc gelatin plus a volatile acid in sufficient quantity to maintain the gelatin in solution despire the presence of the insolubilizing ferric single salts. Such acids are usually formic acid and acetic acid, although apparently one may employ any solubilizing acid or mixture of acids which may be completely or substantially completely driven off the coated gelatinous film at drying temperatures which do not adversely affect the gelatin. The single ferric salts which would normally insolubilizc the gelatin in such a coating mixture are, for example, ferric sulphate, ferric chloride, ferric nitrate, ferric oxalate, ferric citrate, etc.; it is to be noted that these may be salts of either organic or inorganic acids; they may or may not be heretofore recognized lightsensitive ferric salts, although their behavior in this invention indicates that, in fact, they are; possibly the apparent absence of lightsensitivity of some of such single salts heretofore has been due to the extreme slowness of reaction to light of such salts alone or the inhibition of reaction to light in the different environments heretofore encountered. The weight of the mixed single and double ferric salts in above coating systems may range in proportion from approximately 20 percent to percent of the weight of the anhydrous gel material, and, in any such mixture of salts, the proportion of single salts may range, by weight, from approximately 40 percent to 60 percent of the weight of the double salts.

In addition to the mixed solubilizing and insolubilizing salts and volatile acid in the above-described coating solution, one usually adds a non-reacting toning pigment or dyestuff. This latter may be omitted, for example, if the visibility of the ultimate wash-off relief is actually immaterial to the end use, as for example, where the relief image is transferred to block off areas in a screen printing stencil; even for such end uses,

however, a lightly tinctured small amount of a copper phthalocyanine dyestuff, for example, is added to provide a color contrast between the residual image which remains on the base and the areas which have been washed off and to serve as an indication that the washoff has been completed. The amount of toner added to the coating solution should, of course, be insufficient to render the resultant coating substantially nontranslucent.

For most uses the base support for the abovedescribed gelatinous coating is preferably a flexible transparent and relatively dimensionally stable film, usually a polyester [Mylar or cellulose acetate, although other inexpensive films, such as polyethylene or polypropylene, vinyls, etc., are not excluded, and, where extreme toughness is desired, nylon and halogenated (e.g., Teflon) films may be employed. If the end use requires a relatively permanent support for the gelatinous coating, adhesion promoters, such as a gelatin subbing for photographic films, or surface treatment to promote adhesion (e.g., partial surface oxidation of olefin films) should be employed. If the end use requires transfer of the wash-off relief image either an intermediate release coat is employed or the adhesionpromoting treatment of an otherwise poorly adherent supporting film is omitted. Flexibility of production sizes and reduction of storage space usually dictates the use of films as the support for the gelatin coating, but does not exclude the use of rigid supports for the coating, such as glass or rigid plastic sheets; if the end use of the wash-off relief image (suitably tanned or otherwise treated to make it resistant to etching acids) is to provide a resist for making etched metal printing plates, the above coating solution may be applied to permeable tissues for transfer to metal sheets or cylinders.

The above generally described coating composition is coated on its support and then dried, the evolution of the solubilizing acid during drying leaving a continuous film of the gelatin insolubilized (with respect to water) by the presence of the residual insolubilizing single ferric salts. The thickness of the residual dried coating depends upon the intended end use. Where a wash-off relief image is intended for transfer onto a stencil printing screen, the thickness may be as heavy as l to 2 mils; when the image is to be permanently supported on its base, the thickness usually need be no greater than 0.1 to 0.2 mils.

An alternative to the above described preferred procedure for obtaining a continuous film coating of gelatin containing mixed solubilizing and insolubilizing ferric salts upon a suitable support comprises coating the support with a warm aqueous gelatinous solution and then drying the coating; thereafter the dried film is coated with or immersed in a cold aqueous solution of the solubilizing and insolubilizing ferric salts which is allowed to remain until the salts of the second coating have diffused into the gelatinous coating. The gelatinous coating is then dried. This alternative procedure is generally uneconomic due to the additional processing steps required, particularly where the first gelatinous coating is relatively thick and substantial time is required to achieve adequate diffusion of the ferric salts.

By either of the above procedures, the wash-off relief coating is insolubilized by the presence of the dispersed single ferric salts when it is ready for exposure to light. As indicated at the outset, exposure may be by projection of an image upon the film or contact exposure through a translueency or transparency carrying an opaque copy of the image desired to be reproduced. But whereas, with the conventional negative-image wash-off relief films heretofore available, if the user should receive a negative image as the original rather than the usual positive or the end use requires a positive image of the original copy so that he either had to make a photographic negative of the original copy or plan to make a second wash-off relief from his first, by using film made according to this invention it is immaterial whether the original copy is a negative or positive image, just as it is immaterial whether a positive or negative wash-off relief image is desired.

Exposure is made under light sources conventional for standard negative-image wash-off relief processing. What occurs to the mixed ferric salts dispersed in the gelatin during exposure is not fully understood. Presumably the highly light-sensitive double ferric salts are reduced to their ferrous form, the presumed theory and explanation for the action of light on these salts when dispersed in solubilized gelatin in conventional negative-image wash-off relief systems. But whereas the single ferric salts are normally at best very slightly or slowly light-sensitive, apparently when in proximity to the highly light-sensitive double salts in the insolubilized gelatin coating, they are also catalyzed or kicked of for reduction by the light to their ferrous form. And what is most inexplicable is that the lightsensitivity of the double salts as well as the single salts is apparently heightened so that exposure time (and/or light intensity) for positive, and especially negative, images may be reduced. Still further, a tougher and more durable wash-off relief negative image is obtained; this may be due to the fact that the gelatin is already is relatively insolubilized condition at the time of light exposure (as contrasted with a solubilized condition in conventional negative image systems), the presence of the insolubilizing single salts, the apparent interaction of the single and double salts on each other, or a combination of these and other currently totally unappreciated or unapparent factors. i

To develop an exposed film (or plate) made according to this invention so as to produce a negative of the image imposed upon the gelatinous coating, the filmis immersed in. an aqueous solution of an acid and hydrogen peroxide or an acid and a peroxide former. The unexposed but insolubilized areas are solubilized by diffusion of the peroxide-acid solution, but the light-struck areas are not. Upon washing with warm water, the solubilized, unexposed areas are washed off and the peroxide-acid developer is leached out of the light-struck areas which remain as a tough durable negative image of the original copy imposed on the unexposed film.

To develop an exposed film (or plate) made according to this invention so as to produce a positive of the image imposed upon the gelatinous coating, depending upon the formulation of the coating, thefilm may be immersed in an aqueous solution of the ammonium or alkali metal salts of gelatin solubilizing acids, such as formic or acetic acids plus a soluble salt, such as a nitrate, of alkaline earth metals, particularly stontium and barium. While the reactions of this diffused developer on the gelatinous film is not clear, the result is that the light-struck portions are solubilized and the unexposed portions of the gelatinous coating remain insolubilized so that, upon subsequently washing with warm water to wash-off the light-struck portions and leach out the developer solution from the insolubilized portions not exposed to light, a tough durable positive of the image imposed upon the film remains on the support. i

Thereafter, whether a positive or negative image remains, the wash-off relief image may be put to the various uses as outlined above.

Specific examples of the system as described above are as follows:

Coating solutions Example No. 1

Parts by weight Gelatin [very hard litho grade] 12 Ferric ammonium oxalate 4 Ferric nitrate [Fe(NO 9 H 0] 5 Formic acid 5 Copper phthalocyanamic blue dyestuff [Imperial Color Co. No. X-2688] 10 Water 'lized gelatinous continuous film coating of the requisite thickness and evenness on the base support.

Example No. 2

Parts by weight Gelatin [very hard litho grade] l2 Ferric ammonium oxalate Y 2 Ferric oxalate 3 Formic acid 5 Copper phthalocyanamic blue dyestuff [Imperial Chemical Co. No. X4688] 10 Water 90 This coating solution is prepared and applied to a suitable base support in the same manner as Example To develop a negative image after exposure, using a film or plate coated with a gelatinous coating prepared according to either Example 1 or 2, the film is developed in a negative image developer comprised of an aqueous solution of hydrogen peroxide (3 percent) and orthophosphoric acid (1 percent) or of a peroxide former (5 percent sodium perborate) and orthophosphoric acid (1 percent). The unexposed areas of the gelatinous coating are thereafter washed off in warm water, leaving a blue negative image of the copy imposed during exposure.

To develop a positive image after exposure of a film provided with a gelatinous coating formulated according to Example No. 1, the exposed film is immersed in an aqueous solution containing, per lOO parts of water, 10 parts of strontium nitrate and 5 parts of ammonium formate at room temperature. Apparently these salts react with the ferrous salts in the light-struck areas, but not with the ferric salts in the unexposed areas, to release the acid ions and solubilize the gelatin in the lightexposed areas. Thereafter the film is washed in warm water, to remove the thereby solubilized light-exposed portions and leave a blue positive relief image of the copy imposed during exposure.

To develop a positive image after exposure of a film provided with a gelatinous coating formulated according to Example No. 2, all that is necessary is to wash the exposed film gently in warm water. This alone, for reasons which are not fully understood, is sufficient to solubilize and remove the light-exposed portions while leaving the portions not exposed to light as an insolubilized residual positive image of the copy imposed on the film during exposure.

it is to be understood that the proportion of components given in the foregoing examples, while producing operative results, will be varied by those skilled in the art of working with gelatins. Although animal gelatins are obtainable in various qualitative grades, of which the very hard litho grade is among the most consistent, as a natural material some variation in behavior are characteristic of different batches of the material, even when obtained from the most reliable sources. Proofing of production batches and adjustments of the named components and their operative equivalents can, however, produce wash-off relief films which are consistent in behavior. In the foregoing specific examples, the time and temperature for drying the gelatin coating are not given, since this will vary according to the requisite thickness of coating and the behavior of a specific batch of coating solution, as is well-known in the art. Similarly, the exposure times are not given, since this will depend upon the available light source, the copy being reproduced, and the thickness and lightresponsiveness of the particular film, all characteristics for which adjustments are made by those skilled in the art. Similarly the wash-off time and water temperature must be left to those having a degree of skill in the art. In sum, this invention is not to be restricted to the specific examples given; rather, it is necessary and expected that those skilled in the art will make all necessary variations and adjustments of equivalents in employing the invention as defined in the following appended claims.

What is claimed is:

l. A wash-off relief medium capable of having developed thereon a negative or positive silver-free image of a copy image imposed thereon by light comprising, prior to exposure, a supporting base, a substantially insolubilized gel coated thereon, said gel being capable of being solubilized or insolubilized with respect to water by a reaction involving light-sensitive iron salts, and a mixture dispersed in said gel consisting essentially of one or more light-sensitive ferric double salts of a base and a mild organic acid which does not insolubilize said gel and one or more ferric single salts which insolubilize said gel.

2. A wash-off relief medium as defined in claim 1 in which said gel is a continuous coating film of a dried aqueous solution of gelatin, and said ferric double salts being of the class consisting of ammonium, sodium, potassium, and lithium oxalates, citrates, and tartrates.

3. A wash-off relief medium as defined in claim 2 in which the single ferric salts being of the class consisting of ferric sulphate, chloride, nitrate, oxalate, and citrate.

4. A wash-off relief medium as defined in claim 1, in

which the proportion of mixed salts may vary, by weight, in the order of 20 percent to percent of the weight of the anhydrous gel material, and the portion of single salts varies, by weight, in the order of 40 percent to 60 percent of the weight of the double salts.

5. A wash-off relief medium as defined in claim 1, in which the supporting base is an organic flexible film inert to the coating film.

6. A wash-off relief medium as defined in claim 1, in which the supporting base is a rigid plate inert to the coating film.

7. A wash-off relief medium as defined in claim 1, in which the supporting base is permeable material inert to the coating film.

8. A wash-off relief medium as defined in claim 5, in which the gel coating is sufficiently heavy to exist as a self-sustaining film and is releasably supported on said base.

9. A wash-off relief medium as defined in claim 5 in which said gel coating film is substantially permanently adhered to said supporting base.

10. A wash-off relief medium as defined in claim 1, including a coloring toner dispersed in said coating film in a sufficient quantity to impart a color contrast to said base support and insufficient to render said coating substantially non-translucent.

11. A method of making a wash-off relief image which, at the option of the user, is a negative or positive image of a copy image imposed by light on a wash-off relief medium comprising the steps of providing a wash-off relief medium as defined in claim 1, exposing on the salt-containing gel coating thereof to a copy image to provide light-struck and non-light-struck areas with a sufficient amount of light to allow said salts to react to light, immersing said exposed coating, in case a negative image is desired, in a negativedeveloping solution containing a peroxide and an acid to solubilize said non-light-struck areas, and then washing said coating, whereby the struck areas will remain on the base and the non-light-struck areas will wash off, if said coating has been developed with said negativedeveloping solution to provide a negative of the copy image imposed on said medium, and said light-struck areas may be dissolved off, in the absence of treatment with said negative developing solution, to provide a positive of said copy image.

12. A method of making a wash-off relief image, as defined in claim 11, including the alternative step, in case a positive image is desired, of immersing the exposed gel-coating film, prior to wash-off, in an aqueous positive developer solution of a salt of an alkaline earth metal and a salt of an acid which aids the solubilization of the light-struck portion of said gel coating. 

2. A wash-off relief medium as defined in claim 1 in which said gel is a continuous coating film of a dried aqueous solution of gelatin, and said ferric double salts being of the class consisting of ammonium, sodium, potassium, and lithium oxalates, citrates, and tartrates.
 3. A wash-off relief medium as defined in claim 2 in which the single ferric salts being of the class consisting of ferric sulphate, chloride, nitrate, oxalate, and citrate.
 4. A wash-off relief medium as defined in claim 1, in which the proportion of mixed salts may vary, by weight, in the order of 20 percent to 150 percent of the weight of the anhydrous gel material, and the portion of single salts varies, by weight, in the order of 40 percent to 60 percent of the weight of the double salts.
 5. A wash-off relief medium as defined in claim 1, in which the supporting base is an organic flexible film inert to the coating film.
 6. A wash-off relief medium as defined in claim 1, in which the supporting base is a rigid plate inert to the coating film.
 7. A wash-off relief medium as defined in claim 1, in which the supporting base is permeable material inert to the coating film.
 8. A wash-off relief medium as defined in claim 5, in which the gel coating is sufficiently heavy to exist as a self-sustaining film and is releasably supported on said base.
 9. A wash-off relief medium as defined in claim 5 in which said gel coating film is substantially permanently adhered to said supporting base.
 10. A wash-off relief medium as defined in claim 1, including a coloring toner dispersed in said coating film in a sufficient quantity to impart a color contrast to said base support and insufficient to render said coating substantially non-translucent.
 11. A method of making a wash-off relief image which, at the option of the user, is a negative or positive image of a copy image imposed by light on a wash-off relief medium comprising the steps of providing a wash-off relief medium as defined in claim 1, exposing on the salt-containing gel coating thereof to a copy image to provide light-struck and non-light-struck areas with a sufficient amount of light to allow said salts to react to light, immersing said exposed coating, in case a negative image is desired, in a negative-developing solution containing a peroxide and an acid to solubilize said non-light-struck areas, and then washing said coating, whereby the struck areas will remain on the base and the non-light-struck areas will wash off, if said coating has been developed with said negative-developing solution to provide a negative of the copy image imposed on said medium, and said light-struck areas may be dissolved off, in the absence of treatment with said negative developing solution, to provide a positive of said copy image.
 12. A method of making a wash-off relief image, as defined in claim 11, including the alternative step, in case a positive image is desired, of immersing the exposed gel-coating film, prior to wash-off, in an aqueous positive developer solution of a salt of an alkaline earth metal and a salt of an acid which aids the solubilization of the light-struck portion of said gel coating. 